Updated: Fri Nov. 06 2009 22:08:09

CTV.ca News Staff

Investigators probing the deadliest-ever mass killing at a U.S. military base were attempting Friday to find out what motivated the massacre, which was allegedly perpetrated by an army psychiatrist with strong Muslim beliefs.

Thirteen people were killed and more than 30 others were wounded in the rampage, which broke out at the Fort Hood military base on Thursday afternoon.

Officials said Friday that the death toll could continue to rise as 23 people remained hospitalized -- about half of them in intensive care.

Maj. Nidal Malik Hasan, the suspected gunman, has yet to be charged in the shooting. On Thursday, he was in hospital breathing through a ventilator after being shot several times during the incident.

The suspect fired more than 100 rounds from at least one handgun, which was bought legally before the rampage.

According to some witnesses the gunman shouted "Allahu Akbar!" -- an Arabic phrase for "God is great!" -- before opening fire. However, Lt. Gen. Robert Cone said officials had not yet confirmed the reports.

As families of the victims began identifying the dead to the media Friday, officials searched through Hasan's home, his computer and his garbage.

It was also reported Friday that Hasan cleaned out his apartment in the days leading up to the rampage, telling a neighbour he was going to be deployed on Friday to either Iraq or Afghanistan.

Neighbour Patricia Villa said Hasan, 39, offered her some of his possessions, including a new Qur'an, some frozen food, T-shirts, shelves, briefcases, a desk lamp, and $60 to clean up his apartment, The Associated Press reports.

Jacqueline Harris, 44, another of Hasan's neighbours, said he called her boyfriend before the attack happened.

"He just wanted to thank Willie for being a good friend and thank him for being there for him," Harris said. "That was it. We thought it was just a nice message to leave."

The manager of the apartment complex where Hasan lives said he was recently involved in a spat with another soldier over religion.

He said the soldier had been in Iraq and was upset to learn that Hasan was Muslim.

A bumper sticker that read "Allah is Love" was ripped off Hasan's car and the paint on his car was scratched, said the manager, John Thompson.

In an interview with The Washington Post, Hasan's aunt, Noel Hasan of Falls Church, Virginia, said he had been harassed about being a Muslim in the years after the Sept. 11, 2001, terror attacks, and he wanted out of the Army.

In a statement, Hasan's family said that his alleged actions were "despicable and deplorable" and don't reflect how he was raised.

Officials express grief

U.S. President Barack Obama ordered flags at federal buildings to be at half-mast and said Friday he will attend a memorial service for those killed, though a date has not yet been set for the event.

At a news conference Friday, army officials expressed their condolences to the families.

"This is a kick in the gut, not only for the Fort Hood community, but for the entire army," said Army Chief of Staff Gen. George Casey.

"This is a time for the army family to stand together."

He told reporters that the army has programs to help soldiers who may be suffering from mental health issues.

Obama, meanwhile, has asked for calm.

"We don't know all the answers yet. And I would caution against jumping to conclusions until we have all the facts," he said.

FBI was watching Hasan

As officials probe Hasan's background, new details are beginning to emerge about the suspected shooter.

Among them, that the FBI had begun to investigate Hasan over suspicions he was posting comments to a website that suggested he may have been sympathetic to suicide bombers or other terrorist activities.

It has also emerged that Hasan is from Arlington, Va., and that he had studied at Virginia Tech University, where he was working with soldiers suffering from post-traumatic stress disorder.

Hasan, who had no children, had worked at the Walter Reed Army Medical Center for six years but was transferred to Texas in July.

He was also preparing to be deployed overseas for the first time in his military career.

Army officials say Hasan was not known to be a threat or risk to base personnel, a feeling that was shared by some medical staff.

"I'm not aware of any problems here," said Col. Steve Braverman, the Fort Hood hospital commander. "We had no problems with his job performance."

Base locked down after shooting

The shootings began at the base's Personnel and Medical Processing Office around 1:30 p.m. local time while a graduation ceremony was going on in another part of the base.

Officials say medics who were graduating ran out to help the wounded while still in their caps and gowns.

The office handles administration for soldiers who are about to be deployed to Iraq and Afghanistan.

Fort Hood is the largest base in the U.S. and is home to as many as 50,000 soldiers. The base is located about 150 kilometres south of Dallas.

With files from The Associated Press